you may not realize it but you wing guns are much lower than your sight. So if you set your wing mounted guns to 150, at 150 they will hit lower than what you are aiming at. Extending the range raises up the shot. So if you have them set at 300 and you shoot someone at 150-200, it will be exactly where your sight is.
Another factor is cannons vs machine guns. Of course mg rounds are lighter and will start falling further away.
If you set your convergence to 150, your bullets will cross throught he sight 150yards out. They won't hit low, even with wing-mounted guns. If you set it for 300, the rounds cross through the sight at 300yards. They won't cross through the sight at 150-200.
The deciding factor on the cannons vs machine gun rounds is the projectile velocity, not the weight. As odd as it may sound, the heavier projectile may actually have a flatter trajectory than a lighter projectile (again, depending on the speed, not the weight of the projectile).
Back to the 150yard setting with wing-mounted guns (I know you were just using it as an example)... While most settings are somewhat intuitive and forgiving, the close-in settings (200 or closer) are the opposite of intuitive, and just plain bad for the most part. The 150 yard setting will result in "low" hits out to 150, and then after that they'll be high. Godawful high, actually... 5ft high at 300 yards, and around nine feet high at 800 yards. Not only are they high, but they're way spread out too. So high and spread out that IMO hits at 300 yards and beyond would have a lot to do with luck.
There's probably a "sweet-spot" or at least a "sweet range" with wing-mounted .50's, and IMO it's right at around 300 yards. Convergence settings of less than that can be problematic (as shown), while settings much beyond that don't take full advantage of what the .50's have to offer in the way of "punch" (IMO).
Look at the screenshots near the end of this thread-
http://bbs.hitechcreations.com/smf/index.php/topic,291146.0.html